FGF-8 is a member of the highly conserved fibroblast growth factor family of heparin-binding proteins. They share four common tyrosine kinase receptors, FGFR 1-4, and require the binding of a second surface protein, the ubiquitously expressed heparan sulfate proteoglycan, in order to fully activate these receptors. FGF-8 exists in eight isoforms designated a-h, although only a, b, e, and f are present in humans. This recombinant protein is isoform FGF-8b, which is thought to be the most bioactive form and binds its receptors with the highest affinity. It preferentially activates FGFR4 and the IIIc splice variants of FGFR2 and FGFR3. FGF-8 and the other FGF family members affect the proliferation, differentiation, mobility, and survival of several cell types, including fibroblasts, osteoblasts, smooth muscle cells, and neuroblasts. FGF-8 is active mainly during embryonic development promoting skeletal growth and limb bud formation. Expression in adults is limited to tissues involved in spermatogenesis and oogenesis as well as some cancers. It is the first member of the family to have been identified in breast cancer and is believed to contribute to its progression in an autocrine manner.